This invention relates to the combustion of industrial waste gases having relatively low calorific value, including but not limited to the combustion of waste gases produced in carbon black plants. Recovery of heat (hence energy conservation) and elimination of certain atmospheric pollutants are the desired objectives of this invention.
Disposal of such gases has presented many problems, the solutions for which have ranged from combustors where no supporting fuel is needed to sustain combustion, to those where supporting fuel, normally natural gas, is used to obtain ignition and complete combustion of the waste gas. These systems rely upon baffle walls and refractory checker work in the combustion zone to provide stabilization by heat radiating surfaces. They also rely upon long residence time in combustion chambers of large volume, preheating of combustion air and waste gas, and intensive mixing.
The rapid escalation of fuel prices and shortage of natural gas are strong incentives for development of combustion systems capable of efficiently burning low calorific value gases of varying composition. Using carbon black plant waste gases as an example, waste gases having calorific values varying from about 30-75 BTU/Ft..sup.3 are produced, depending upon the grade of carbon black being produced.
It is particularly desirable to eliminate the need for supplemental (supporting) fuel and to achieve high heat release rates so as to minimize combustor size.
It is also important to minimize the total pressure drop across the system due to the large fans and hence large amounts of power required to pressurize the system.
Cyclone combustors of cylindrical form utilizing a plurality of tangential inlet ports for air and fuel distributed over a substantial part of the length of the cylinder have desirable characteristics for waste gas combustion in that they combine a long residence time with the presence of an aerodynamic reverse-flow/recirculating zone near the outer refractory walls by which heat recirculates to one side of the flame front and heat radiates from the walls to the other side of the flame front. For example, see Agrest, J., "The Combustion of Vegetable Materials & Cotton Husk Combustion Problems," J. Inst. Fuel, Vol. 38, pp. 344-348, 1965; Schmidt, K. R. "The Rotary Flow Furnace of Siemens-Agrest," V.D.I.-Berichte, Vol. 146, pp. 90-101, 1970.
Preliminary experimental work utilizing a cyclone combustor for carbon black plant waste gases is described in a paper presented at the Apr. 21-22, 1975 Joint Meeting of Central and Western States Sections of the Combustion Institute: "The Combustion of Low Calorific Value Waste Gas," by K. R. Dahmen and N. Syred. In this cyclone combustor, the gas/air mixture enters a cylindrical combustion chamber or furnace through a plurality of tangential inlet ports and the outlet of the furnace is of smaller diameter than the diameter of the combustion chamber.
This invention is directed to an improvement on the cyclone combustor described in the Dahmen-Syred paper, whereby the aerodynamics of operation of the combustor can be adjusted so as to minimize the pressure losses across the system for waste gases of relatively low but varying calorific values.
The capability of such a device to burn gases with very low calorific value is improved by increasing the degree of swirl of the tangentially-flowing gases and also by decreasing the ratio of exit diameter to the combustion chamber diameter.
The beneficial effects of the above arrangements are diminished by considerable pressure drop over the system, and the improvements in combustion characteristics are sometimes hard to realize without incurring unacceptably high pressure losses.
Analysis of the pressure losses show that one part of these losses is directly related to the velocity in the tangential inlet ports, independent of conditions within the combustion chamber. This tangential inlet velocity is also an important factor in the magnitude of the Swirl Ratio. The Swirl Ratio is hereinafter quantified by Swirl Number, "S." The second part of the pressure losses is determined by the flow velocity in the furnace and especially in the restricted outlet.
If the furnace could be designed for a narrow range of waste gas quality, a combination of dimensions for inlets and outlets could be chosen to provide the optimum combustion characteristics commensurate with an acceptable pressure loss. In the majority of applications in a carbon black plant, however, waste gases having a wide range of qualities will have to be burned. The combustor will have to be designed for waste gas of the lower quality level with respect to tangential inlet velocity for Swirl Number and outlet velocity in the restricted exit throat. Such a design then will involve the highest acceptable system pressure drop. However, when grades of carbon black are produced which yield a waste gas of higher heating value, the temperatures in the furnace and in the outlet are much higher resulting in increased combustion chamber drag and outlet velocity, increasing the pressure drop significantly. As a result, the provisions to overcome the pressure losses have to be greater than would be required for the burning of the low calorific waste gas.
As suggested above, the leaner waste gases of very low calorific value require use of a higher Swirl Number than the richer waste gases having higher calorific values. Inasmuch as the Swirl Number is directly proportional to the tangential inlet velocity, the tangential inlet velocity can be reduced when richer gases are burned, thus reducing the inlet pressure losses so as to balance or substantially balance the increases in combustion chamber drag and outlet pressure loss. Such a balanced design not only increases efficiency but reduces capital investment by reducing the required maximum design capacity or capacity of the cyclone combustor and of the fans needed for pressurizing the system.